THE GERM-PLASM THEORY 307 



smaller elements. These differently sized dyads are 

 considered by Carothers as "distinct physiological 

 individuals, representing respectively the paternal 

 and maternal contribution to the formation of some 

 character or characters ; and, as each can be iden- 

 tified, they furnish an excellent means of tracing the 

 process of segregation and recombination" (p. 499). 

 It was at first assumed that each of the pairs of 

 chromosomes which unite in synapsis was responsible 

 for a single adult 

 character, but a> 

 the number of d< 

 Mendelian char- (^"^3 0' 

 acters is known 



.1 . FIG. 83. Arphia simplex. Chromosomes of 



T.O DC greater in g rs ^. S p e rmatocyte. a = accessory chromo- 

 Certain Cases than some, b = unequal dyad. (From Carothers, 

 1913.) 



the number of 



chromosomes. Fortunately, it has been found that 

 the characters, instead of undergoing independent as- 

 sortment, may become linked so that certain of them 

 almost always occur together in the offspring. The 

 relation of these facts to the constitution of the 

 chromosomes may best be illustrated by reference to 

 the studies of Morgan and his students on the fruit- 

 fly, Drosophila. Over one hundred mutants of this 

 species have been discovered by these investigators. 

 So far as studied, the characters of these flies seem to 

 form three groups. ' ' The characters in the first group 

 show sex-linked inheritance. They follow the sex- 

 chromosomes. The second group is less extensive. 

 Since the characters in this group are linked to each 



